Hundreds of the Peninsula Valdés whales have died since monitoring of their population size began in 1971, researchers report in astudy published this month in the journal Marine Ecology Progress Series. From 1971 to 2011, 630 of the right whales died—adults and young ones combined.

But 77 percent of those deaths occurred between 2003 and 2011. And of those recent deaths, 89 percent have occurred in the calves. Scientists are still struggling to understand why.